2599: [IOI2011]Race

2599: [IOI2011]Race

链接

分析

　　被memset卡。。。

　　点分治，对于重心，遍历子树，记录一个数组T[i]，表示以重心为起点的长度为i的路径中最少的边数是多少。然后先遍历子树，更新答案，然后在遍历一边更新T，防止出现两个起点在同一棵子树中的情况。

代码

 #include<cstdio>
 #include<algorithm>
 #include<cctype>

 using namespace std;

 const int N = ;
 const int INF = 1e9;

 struct Edge{
     int to,nxt,w;
     Edge() {}
     Edge(int a,int b,int c) {to = a,w = b,nxt = c;}
 }e[N<<];
 int head[N],siz[N],T[];
 bool vis[N];
 int n,k,tot,Size,Num,Root,Ans = N;

 inline int read() {
     int x = ,f = ;char ch=getchar();
     for (; !isdigit(ch); ch=getchar()) if(ch=='-')f=-;
     for (; isdigit(ch); ch=getchar()) x=x*+ch-'';
     return x*f;
 }
 void add_edge(int u,int v,int w) {
     e[++tot] = Edge(v,w,head[u]);head[u] = tot;
     e[++tot] = Edge(u,w,head[v]);head[v] = tot;
 }
 void getRoot(int u,int fa) {
     siz[u] = ;
     int mx = ;
     for (int i=head[u]; i; i=e[i].nxt) {
         int v = e[i].to;
         if (v == fa || vis[v]) continue;
         getRoot(v,u);
         siz[u] += siz[v];
         mx = max(siz[v],mx);
     }
     mx = max(Size-siz[u],mx);
     if (mx < Num) Root = u,Num = mx;
 }
 void dfs1(int u,int fa,int L,int C) {
     if (L > k) return ;
     Ans = min(Ans,T[k-L]+C);
     for (int i=head[u]; i; i=e[i].nxt) {
         int v = e[i].to;
         if (v == fa || vis[v]) continue;
         dfs1(v,u,L+e[i].w,C+);
     }
 }
 void dfs2(int u,int fa,int L,int C,int f) {
     if (L > k) return ;
     if (f) T[L] = min(T[L],C);
     else T[L] = INF;
     for (int i=head[u]; i; i=e[i].nxt) {
         int v = e[i].to;
         if (v == fa || vis[v]) continue;
         dfs2(v,u,L+e[i].w,C+,f);
     }
 }
 void calcc(int u) {
     T[] = ;
     for (int i=head[u]; i; i=e[i].nxt) {
         int v = e[i].to;
         if (vis[v]) continue;
         dfs1(v,u,e[i].w,);
         dfs2(v,u,e[i].w,,);
     }
     for (int i=head[u]; i; i=e[i].nxt) {
         int v = e[i].to;
         if (vis[v]) continue;
         dfs2(v,u,e[i].w,,);
     }
 }
 void solve(int u) {
     vis[u] = true;
     calcc(u);
     for (int i=head[u]; i; i=e[i].nxt) {
         int v = e[i].to;
         if (vis[v]) continue;
         Size = siz[v],Root = ,Num = 1e9;
         getRoot(v,);
         solve(Root);
     }
 }
 int main() {
     n = read(),k = read();
     for (int i=; i<=k; ++i) T[i] = INF;
     for (int u,v,w,i=; i<n; ++i) {
         u = read(),v = read(),w = read();
         add_edge(u+,v+,w);
         if (w == k) {printf("-1");return ;}
     }
     Size = n;Num = 1e9;
     getRoot(,);
     solve(Root);
     if (Ans == N) puts("-1");
     else printf("%d",Ans);
     return ;
 }